The term device-to-device (D2D) used in the following corresponds to any direct transmission occurring between two or more devices in a cellular devices, i.e. not via one or more network nodes or other network elements, such as an eNodeB, a backbone network, etc., for the purpose of e.g. direct control signaling, direct data communication or peer device presence discovery.
Although the idea of enabling D2D communications as a means of relaying in cellular networks was proposed by some early works on ad hoc networks, the concept of allowing local D2D communications to (re)use cellular spectrum resources simultaneously with ongoing cellular traffic is relatively new. Because the non-orthogonal resource sharing between the cellular and the D2D layers has the potential of reuse gain and proximity gain, at the same time increasing the resource utilization, D2D communications underlying cellular networks has received considerable interest in the recent years.
Specifically, in 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) networks, such as LTE Direct, i.e. employing D2D, communication can be used in commercial applications, such as cellular network offloading, proximity based social networking, or in public safety situations in which first responders need to communicate with each other and with people in the disaster area. See for example the specification 3GPP TR 22.803, V1.0.0, 2012-08.